1. Field of the Invention
The present invention relates generally to geophysical exploration. In another aspect, the invention concerns a method for designing and conducting a 3-D seismic survey that records converted wave data.
2. Description of the Prior Art
Seismic surveying is a key technology area in geophysical prospecting. In a typical seismic survey, a source (e.g., vibroseis or explosive) on the surface of the earth generates a signal which propagates through the earth. The subsurface geological structures attenuate, reflect, and refract the signal. Receivers (e.g., geophones) on the earth""s surface monitor the reflected wave. From the traces gathered by the receivers, seismologists construct a model of the earth""s subsurface.
Over the years, increasingly sophisticated survey designs and data processing techniques have been developed. For example, until the early 1980s, most land seismic surveys were two-dimensional (2-D) surveys conducted along a single line of source and receiver points. Today, most surveys are three-dimensional (3-D) where sources and receivers are scattered along a plane and the geometry of the sources and receivers are defined by the requirements of the survey.
Seismic energy can propagate through the earth in one of two forms: compressional waves (P-waves) and shear waves (S-waves). P-waves have vector displacements parallel to the direction of propagation, whereas S-waves have vector displacements that are orthogonal to the direction of propagation. xe2x80x9cConverted wavesxe2x80x9d travel first as one type of wave and then the other, with the conversion between wave types happening at seismic discontinuities. If the conversion is from an incident P-wave to a reflected S-wave at the reflecting geological structure, this reflection mode is called a C-wave. S-waves travel through the earth with a velocity dependent on the shear rigidity of the subsurface formation. Thus, S-waves contain different information about the subsurface structure than do P-waves.
Although all seismic surveys generate C-waves, ordinary techniques of seismic signal reception and processing are designed to suppress these C-waves in favor of P-wave arrivals. Nonetheless, there are many exploration and exploitation settings wherein one would wish to maximize, rather than suppress, C-wave arrivals (e.g., where the target cannot be readily imaged by P-waves). This might happen, for example, where the elastic contrasts of the subsurface rock layers yield only weak P-wave reflections; where salt bodies occur above the target; or, where subsurface xe2x80x9cgas cloudsxe2x80x9d obscure the target, as might occur in connection with a hydrocarbon reservoir above which the over-burden contains a small concentration of gas. In this latter situation, the gas may severely delay and attenuate conventional P-waves traveling through the overburden, so that the underlying reservoir will be poorly imaged. However, a gas-filled rock does not unduly slow or attenuate S-waves, so one may be able to obtain better images of such reservoirs using C-wave techniques.
Conventional seismic processing relies heavily on a stack (or average) of seismic traces from a common midpoint (CMP) gathered to reduce noise in the seismic section, and as a tool for estimating subsurface velocities. The stacking approach is generally satisfactory for P-wave seismic data, but often fails when applied to C-wave data. One reason for this is that the travel paths of C-waves are asymmetrical, even for simple horizontally layered medium. Multiple coverage of the same subsurface point cannot be achieved for C-wave data by stacking a CMP together, but instead requires true common reflection point (CRP) sorting which, for C-wave reflections, is actually a common conversion point (CCP) gather.
Many studies have focused on methods for processing and interpreting seismic data to provide a better image of subsurface formations. However, much less attention has been given to enhancing the efficiency of seismic surveys by optimizing the geometric design (i.e., layout) of the seismic sources and receivers, while still maintaining the desired geophysical requirements of the survey. This is particularly true for 3-D C-wave surveys where the travel paths of C-waves are asymmetrical.
It is, therefore, an object of the present invention to provide a novel method of designing a seismic survey layout wherein the number of seismic survey components (i.e., seismic sources and receivers) is minimized, while still maintaining the desired geophysical requirements of fold, azimuth, and offset.
A further object of the present invention is to provide an improved method for conducting a 3-D C-wave seismic survey with minimal cost and optimum subsurface illumination.